1. Field of the Invention
This invention relates to fabrication of semiconductor devices. More particularly, this invention relates to the detection of subsurface defects in non-homogeneous structures such as multilayered integrated circuits.
2. Description of the Related Art
Semiconductor structures are inspected prior to, during, and after patterning procedures. Patterned metal films used in integrated circuit devices are often created using a damascene technique, in which a pattern is etched in an insulating dielectric layer, and subsequently filled using any of several standard deposition techniques, e.g., chemical vapor deposition (CVD), physical vapor deposition (PVD), or electro-copper plating (ECP). In the course of this process defects may be created inside or under the metal, such as voids, delamination, underfill or underetch of the dielectric, and other interface-related defects.
Generally, these defects are not directly accessible using optical inspection techniques due to opacity of the surface layer. To some degree, they may be detected using voltage-contrast scanning electron microscopy (SEM), or electron beam inspection (EBI). Although buried defects can sometimes be seen using SEM, that technique is better adapted for evaluation of particular positions on a substrate or wafer, rather than for scanning. EBI is an inspection technique employing a scanning electron beam, which can scan significant portions of an entire wafer to automatically find defects. It can in principle see whatever a SEM tool sees. However, EBI tools are generally too slow and expensive for the production floor. They are typically used in the research and development stage of product development.
It is proposed in U.S. Pat. No. 4,710,030 to employ a pump beam of short, non-destructive laser pulses (0.01–100 ps duration) to induce a thermo-elastic deformation, or stress waves, in a structure being tested, and to monitor the transient response of the structure using a low-power laser probe beam that is directed to the area of the deformation. By analyzing the intensity of the returning probe beam, information regarding defects and other characteristics of the structure can be inferred.
Besides reflections of short-pulse-induced stress waves, voids and interface defects are known to produce other physical effects in response to a pump beam, such as changes in acoustic dispersion properties, and reduced heat dissipation. These effects are discussed in the document Pico-second Ultrasonics, Grahn et al., IEEE Journal of Quantum Electronics, Vol. 25 No. 12, pp. 2562–2568 (December 1989).
U.S. Pat. No. 5,633,711 discloses another example of monitoring the transient response to an excitation laser pulse that impinges on and locally heats a structure. In this disclosure, besides the intensity of the probe beam, phenomena such as acoustic oscillations and polarization disturbances are taken into account.
A disadvantage of the techniques disclosed in the above-noted patents is a low signal-to-noise ratio (SNR) in the detection signals. The stress wave produced by the pump beam is associated with very small changes in reflectivity (expressed as a percent of the light falling on the surface). Values from 1×10−6 to 1×10−4 are typical. It has thus been necessary to compensate for the poor SNR by repeating the detection sequence over a relatively long period, for example, a second for each detection spot. Many repetitions of the detection sequence are generally required to obtain meaningful data. Furthermore, the frequency of repetition is itself limited by the need to perform mechanical adjustments in the detection unit between performance cycles. Thus, the time needed to evaluate a structure becomes impracticably long for full-wafer inspection purposes.
U.S. Pat. No. 6,320,666 discloses an intensity modulated pump laser beam, which is focused onto a sample so as to excite the sample periodically. Periodic heating by the pump beam creates a time varying deformation in the sample surface. A probe laser beam, obtained from a second laser, is focused onto the sample within the periodically heated area. The pump and probe beam are spaced apart, and the probe beam is said to undergo periodic angular deviations at the frequency of the modulated heating. A photodetector is provided for monitoring the reflected power of the probe beam and generating an output signal responsively thereto. The output signal is filtered and processed to provide a measure of the modulated optical reflectivity of the sample. A steering apparatus is provided for adjusting the relative position of pump and probe beam spots on the sample surface. The steering apparatus is used to move the beam spots from an overlapping, aligned position, to a position of separation of up to about 10 microns. Measurements can be taken as the separation of the beam spots is gradually changed, or at discrete separation intervals. It is also proposed to increase information by varying the modulation frequency of the pump beam, and to obtain independent reflectivity measurements at a plurality of wavelengths using a polychromatic light source.
U.S. Pat. No. 5,748,317 discloses the use of laser time-delayed pump and probe beams for determining the thermal properties of thin film. Measurements of reflectance and other optical characteristics are used to estimate the Kapitza resistance of a film. Inferences regarding the structure of the film or interfaces therein are made using reference data obtained from simulation or from another sample. The technique requires quantitative conclusions to be drawn about a sample. This is relatively time-consuming and complex, and is not ideal for the rapid qualitative evaluation of production line output, which large wafers or similar specimens may need to be quickly evaluated.
It is proposed in U.S. Pat. No. 6,253,621 to analyze acoustic waves that are generated in a sample under test in response to a pulsed laser that is directed to a micro-spot on the sample and scanned. Acoustic waves are detected, and an acoustic index of refraction of a portion of the conductive structure is calculated as a function of the wave. The acoustic index of refraction is then spatially mapped over the sample. It is asserted that defects can be detected by intra-sample comparisons, or by comparison with a device that is known not to be defective.
The above-noted conventional techniques require extensive analysis of time-dependent signals. They are slow and computationally expensive.